1. Field of the Invention
One aspect of the present invention relates to high-strength, non-agglomerated porous uniform microspheres of silica from 1.0 to 50 microns in size which are produced by spray drying a mixture comprising a colloidal silica sol and an effective amount of an additive selected from ammonium citrate or urea.
Another aspect of the present invention relates to an attrition resistant catalytic composite consisting essentially of metal crystallites distributed throughout said silica microspheres and method for preparing the same.
A further aspect of the present invention relates to an improved process for making hydrogen peroxide from the direct combination of hydrogen and oxygen in the presence of said catalytic composite consisting essentially of metal crystallites such as palladium and platinum-palladium distributed throughout high-strength, attrition resistant, non-agglomerated porous silica microspheres.
2. Description of the Related Art
The preparation of spheroidal ceramic particles by the process known as spray drying is well known as will be apparent from the ensuing discussion of prior art. Spray drying is advantageous in that it constitutes an economical preparation process. However, conventional spray drying of silica aquasols often produces hollow microspheres having a large core hole. Frequently, the spherical form is distorted due to rupturing of the microspheres during the spray drying process which produces amphoras or donut type particles as described in greater detail in K. Masters, "Spray Drying Handbook", chp. 8, Leonard Hill Books, London (4th ed. 1985). An impervious crust can form around each droplet during the process before drying is complete. When the crust becomes non-porous, vapor pressures within each droplet can build up until the droplet ruptures, forming the donut or amphora, or it can even disintegrate. The present invention provides for spray drying silica aquasols which avoids formation of an impervious crust around the droplets. Unique microspheres result which when calcined give high strength porous microspheres. These products are useful as commercial chromatographic column packing materials and high strength catalytic supports useful in fluid bed and slurry applications. More particularly, the products are useful as high strength catalytic supports in the catalytic process for the direct combination of hydrogen peroxide.
U.S. Pat. No. 4,131,542 describes a process for making amorphous silica grains which involves spray drying an aqueous silica sol to form micrograins. The micrograins are then acid washed and sintered.
U.S. Pat. No. 4,389,385 describes a process for preparing a porous gel of an inorganic material, such as silica, by dispersing solid primary particles of the material in a liquid, e.g., water, to form a sol containing colloidal particles which are aggregates of the primary particles and then spray drying the sol to form the gel. It is noted that the inorganic materials, i.e., the solid primary particles, for which the process is designed are derived from a vapor phase condensation method, such as by flame hydrolysis of volatile halides or alkoxides. The process requires aggregation of the primary particles prior to spray drying, such that each aggregate contains, for example, somewhere in the range of 1.5.times.10.sup.3 particles or more. The range of mean pore diameter in the examples is 28 to 120 nm, which is very large in comparison with the primary particle size of the starting sol, 7 to 40 nm. Such an open structure would not be strong enough for process scale chromatographic columns or catalytic reactors.
Japanese Patent Publication 61-174,103 describes a process for producing porous spherical fine powders having an average particle size 1-20 microns from spray drying a mixture of a colloidal oxide sol and an inorganic oxide gel. It is suggested that the sol acts as a good binder for the gel resulting in the production of spherical particles which are not obtained using a gel alone. Even with both components present the spray drying conditions are quite critical and a temperature greater than 150.degree. C. causes a breakdown in the balance between the shrinkage rate and the drying rate and non-spherical particles and cracked particles result. The process of the invention described hereinafter does not require the prior mixing of sol and gel, and a broad range of spray drying conditions is applicable. A considerably narrower particle size distribution and pore size distribution is obtained using the process of the present invention, which is particularly advantageous when the microspheres or granules produced therefrom are to be granules used as packing in chromatographic columns and catalytic applications.
In the direct combination of hydrogen and oxygen to form hydrogen peroxide, catalysts comprising metals supported on silica are documented in the art. For example, U.S. Pat. Nos. 4,772,458 and 4,832,938 describe amphora or donut type silica supported platinum and palladium and varying platinum-palladium compositions catalysts, which are prepared by spray drying a colloidal silica sol containing colloidal Pt, Pd or varying Pt/Pd compositions or their metal salts. The catalytic composite of the present invention exhibits high-strength and improved attrition resistance, and can be produced by spray drying a mixture comprising colloidal silica sol, an additive selected from ammonium citrate or urea, and metal salt or metal salts; alternatively, the catalytic composite can be produced by applying a metal salt or metal salts to said pre-formed silica microspheres. Attrition resistance is defined herein as the useful life of the catalyst, i.e., resistance of catalyst to mechanical degradation.
It has been discovered in the present invention that the catalytic composite demonstrates superior performance in the direct combination of H.sub.2 and O.sub.2 to hydrogen peroxide to the catalysts disclosed in the prior art. Microspheres also provide an excellent support for a wide range of metal catalysts which are used in many processes and which are described in the literature as being supported on a variety of materials, including silica. Such catalysts when brought into contact with reactants in agitated slurries, or fluidized beds are susceptible to deterioration by attrition. This decreases their activity over time and they have to be replaced more often because the catalyst breaks up into fine fragments. These fine fragments result in filtration problems as well. Clearly it is is advantageous to increase the effective life of such catalysts. There is a strong economic incentive to develop process improvements and this has been accomplished by the supports of the present invention.